Potassium Nitrate → 4,6-dimethyl-5-nitro-2,3-dihydro-1H-indole-2,3-dione

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The method of Cheesman, G. W. H.; J. Chem. Soc. 1170 (1962), was adapted. 4,6-Dimethylisatin (89 mg, 0.50 mmol) was dissolved in concentrated H2SO4 (1.0 mL) at 0° C. for 30 min with stirring, then KNO3 (55 mg, 0.55 mmol, Baker) was added to this solution. The mixture was stirred at 25° C. for 24 h. It was then poured into ice water (5 g). The resulting precipitate was collected by filtration. The precipitate was dissolved in 1N KOH (5 mL) and the small amount of solid was removed by filtration, then the filtrate was acidified to pH 2 with 4N HCl (~1 mL) to give a yellow precipitate. The solid was collected by filtration and was washed with water (2×1 mL), then it was dried in the air at 50° C. for 4 h affording pure (by NMR) title compound (85 mg, 77%) as a yellow powder. Crystallization from acetone/water gave crystals. Mp 278°-80° C. (dec.). IR (KBr, cm-1): 3452, 3225, 2931, 1780, 1742, 1635, 1608, 1535. 1H NMR (DMSO-d6): δ 11.357 (s, 1H), 6.771 (s, 1H), 2.389 (s, 3H), 2.305 (s, 3H). Purity >97.5% by HPLC.

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